1. Field of the Invention
The present invention relates generally to neutron absorbing materials used in spent nuclear fuel storage and shipping containers and particularly to such containers having homogeneous extruded borated aluminum neutron absorbing rodlets interspersed in empty cells of the spent fuel assembly.
2. Description of the Prior Art
In a nuclear reactor, the nuclear fuel is usually in the form of a plurality of individual rods assembled into a bundle of substantially square cross-section, in a manner that the rods are held in fixed, spaced relationship in a plurality of cells. Some of these cells are guide tubes containing control rods and instrumentation. One assembly design is a square bundle of cells in a 15.times.15 array with 17 of these cells used for control rod and instrumentation insertion.
Over a prolonged period of operation of the reactor, the fissionable fuel becomes depleted to the point where it is no longer efficient for continued use. When this state is reached, the spent fuel assembly is removed and replaced. When the spent fuel assembly is removed from the reactor the control rods and instrumentation are retracted leaving empty cells at these locations. The depleted fuel assembly is still of potential value, since the fuel rods are still reactive and can be reprocessed in a suitable facility to become capable of sustaining or fueling a nuclear reaction.
Reprocessing facilities are usually far removed from the nuclear reactor and it is necessary to ship the spent fuel over long distances, in as safe a manner as possible, for both the outside world and the fuel assembly itself. To ensure the necessary degree of safety required, the fuel assembly is loaded into a shipping cask. It is imperative that the cask assembly be so constructed that harmful radiation does not escape, that the heat generated by the radioactive decay of the spent fuel is adequately dissipated, and that the interaction between the fuel cells is such that criticality safety criteria are not violated. To achieve these ends numerous types of fuel cell shipping containers have been designed and used. The most common have been circular casks into which the fuel bundle assembly is placed. All such containers depended upon nuclear shielding located around the circumference of the cask to prevent radiation leakage during shipment or storage.
One example of such a cask is described in U.S. Pat. No. 4,827,139 wherein rectangular filler blocks or spacers are located between nuclear fuel compartments of a spent nuclear fuel shipping basket. These components are extruded from a neutron poison material such as an alloy of aluminum and Boron. It also suggests that filler blocks which define other shapes such as triangles could also be made from such material. However, there are no teachings concerning use of extruded rodlets as reactivity controllers nor of the placing of such rodlets directly into the empty assembly cells previously used for control rods and instrumentation.
Another example of such a cask is found in U.S. Pat. No. 5,373,540 wherein cruciform components are placed between spent nuclear fuel assemblies in a shipping basket that is located within the cask. The components are made from a Boron aluminum alloy. Again, there is no teaching of using rodlets as a reactivity control mechanism by placing them directly into the control rod or instrument cells rather than making the cell compartments of such reactivity control material.
U.S. Pat. No. 4,143,276 teaches yet another type of spent fuel container wherein the sides of the individual fuel assembly holders are made from reactivity control material.
It should also be mentioned that spent fuel assemblies are first stored at the reactor site pending shipment in shielded casks to recycling or storage facilities. Usually such storage is in nuclear shielded water filled or sometimes dry facilities. The entire fuel bundle assembly with empty control rod and instrumentation cells is usually stored in these facilities without any neutron absorbing material in these empty cells. Thus, an effective neutron absorbing material can be located into the empty cells of a spent fuel assembly during in-house storage as well as later shipment to a fuel reprocessing facility.